In recent years, in response to steep rise in crude oil price and to depletion of petroleum oil resources anticipated in the near future, deeper oil fields, which have not be taken into consideration in the past, very corrosive sour gas fields, the development of which was abandoned once in the past, and the like have been aggressively developed on a worldwide basis. The oil fields and gas fields as described above are generally located in very deep places, and in addition, these oil and gas fields are in a very severe corrosive environment in which the temperature is high and CO2, Cl−, and the like are present. Hence, as an oil-well steel pipe used for mining oil and gas fields as described above, a steel pipe having high strength and also having superior corrosion resistance is required.
Heretofore, in oil wells and gas wells in an environment containing CO2, Cl−, and the like, 13% Cr martensite stainless steel pipes, which have superior CO2 corrosion resistance, have been generally used as an oil-well steel pipe. However, there has been a problem in that a general martensite stainless steel cannot withstand the use in an environment in which a large amount of Cl− is present and the temperature is high, such as more than 100° C. Hence, in a well in which steel pipes and the like are required to have corrosion resistance, a dual phase stainless steel pipe has been used. However, since the dual phase stainless steel pipe contains a large amount of alloy elements, hot workability thereof is not superior, and hence a specific hot working can only be used for forming the dual phase stainless steel pipe, thereby causing the increase in cost. In addition, when the yield strength of a conventional 13% Cr martensite stainless steel pipe is more than 654 MPa, the toughness thereof is seriously degraded, and hence there has been a problem in that the 13% Cr martensite stainless steel pipe may not be used.
In addition, in recent years, development of oil wells in cold regions has been increasingly carried out. Hence, besides high strength, superior low-temperature toughness has also been required for the steel pipe in many cases.
According to the situations described above, a high strength 13% Cr martensite stainless steel pipe for use in oil wells has been strongly desired, which is primarily formed of inexpensive 13% Cr martensite stainless steel having excellent hot workability and which has a high yield strength of more than 654 MPa (95 ksi), superior CO2 corrosion resistance, and a high toughness.
In response to the requirements described above, for example, in Japanese Unexamined Applications 8-120345, 9-268349 and 10-1755 and Japanese Patents 28-14528 and 32-51648, improved martensite stainless steel or a steel pipe thereof have been proposed which are obtained by improving the corrosion resistance of 13% Cr martensite stainless steel or a steel pipe thereof.
A technique disclosed in Japanese Unexamined Application 8-120345 is a method for manufacturing a martensite stainless steel seamless pipe having superior corrosion resistance. According to the method described above, after a 13% Cr stainless-steel raw material having a composition in which the content of C is controlled in the range of 0.005% to 0.05%, 2.4% to 6% of Ni and 0.2% to 4% of Cu are collectively added, 0.5% to 3% of Mo is further added, and a Nieq is adjusted to 10.5 or more is processed by hot working, cooling at a rate faster than that of air cooling is performed. Heating may further be performed to a temperature in the range of (the Ac3 transformation point+10° C.) to (the Ac3 transformation point+200° C.) or may further be performed to a temperature in the range of the Ac, transformation point to the Ac3 transformation point, followed by cooling to room temperature at a cooling rate faster than that of air cooling, so that tempering is performed. According to the technique described in Japanese Unexamined Application 8-120345, a martensite stainless steel seamless pipe can be manufactured which simultaneously has a high strength equivalent to or more than that of API-C95 grade, corrosion resistance in an environment at 180° C. or more containing CO2, and the SCC resistance.
A technique disclosed in Japanese Unexamined Application 9-268349 is a method for manufacturing a martensite stainless steel having superior resistance to sulfide stress cracking. According to the method described above, after 13% Cr martensite stainless steel having a composition in which 0.005% to 0.05% of C and 0.005% to 0.1% of N are contained, and in which Ni, Cu, and Mo are controlled in the ranges of 3.0% to 6.0%, 0.5% to 3% and 0.5% to 3%, respectively, is processed by hot working, followed by spontaneous cooling to room temperature, heating is performed to a temperature in the range of (the Ac1 point+10° C.) to (the Ac1 point+40° C.), and the stainless steel is held for 30 to 60 minutes at that temperature and is then cooled to a temperature to the Ms point or less. Subsequently, tempering is performed at a temperature of the Ac1 point or less, so that a texture is formed in which tempered martensite and 20 percent by volume or more of a γ phase are both present. A tempered martensite texture containing 20 percent by volume or more of a γ phase is formed, the resistance to sulfide stress cracking is significantly improved.
According to a technique described in Japanese Unexamined Application 10-1755, martensite stainless steel has a composition containing 10% to 15% of Cr in which the content of C is controlled in the range of 0.005% to 0.05%, 4.0% or more of Ni and 0.5% to 3% of Cu are collectively added, 1.0% to 3.0% of Mo is further added, and in addition, the Nieq is controlled to−10 or more. By performing tempering, a texture is formed containing a tempered martensite phase, a martensite phase, and a retained austenite phase so that the total fraction of the tempered martensite phase and the martensite phase is set to 60% to 90%, thereby obtaining martensite stainless steel having superior corrosion resistance and resistance to sulfide stress cracking. The corrosion resistance and resistance to sulfide stress cracking in a wet carbon dioxide gas environment and in a wet hydrogen sulfide environment are improved.
A technique described in Japanese Patent 28-14528 relates to a martensite stainless steel material for use in oil wells, having superior resistance to sulfide stress cracking, the stainless steel material having a steel composition in which more than 15% to 19% or Cr is contained, 0.05% or less of C, 0.1% or less of N, and 3.5% to 8.0% of Ni are contained, and 0.1% to 4.0% of Mo is further contained, and in which 30Cr+36Mo+14Si−28Ni≦455 (%) and 21Cr+25Mo+17Si+35Ni≦731 (%) are simultaneously satisfied. A steel material having superior corrosion resistance in a severe oil well environment in which chloride ions, a carbon dioxide gas, and a small amount of a hydrogen sulfide gas are present.
A technique described in Japanese Patent 32-51648 relates to a precipitation hardened martensite stainless steel having superior strength and toughness, the stainless steel having a steel composition in which 10.0% to 17% or Cr is contained, 0.08% or less of C, 0.015% or less of N, 6.0% to 10.0% of Ni, and 0.5% to 2.0% of Cu are contained, and 0.5% to 3.0% of Mo is further contained, and having a texture in which, owing to a cold working of 35% or more and annealing, the average crystal particle diameter is set to 25 μm or less and the number of precipitates, which are precipitated in a matrix and which have a particle diameter of 5×10−2 μm or more, is reduced to 6×106/mm2 or less. Since a texture is formed containing fine crystal particles and having a small amount of precipitates, precipitation hardened martensite stainless steel, which has a high strength and causes no decrease in toughness, can be provided.
However, there has been a problem in that improved 13% Cr martensite stainless steel pipes manufactured by the techniques discussed above cannot stably exhibit desired corrosion resistance in a severe corrosive environment in which CO2, Cl−, and the like are present and the temperature is high, such as more than 180° C.